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Every year around Christmas we receive a visitor from the Nordic countries. A hawk keeps returning to a particular branch of a single tree throughout the entire winter, knowing that he will obtain daily treats with little effort. With the advent of spring, he happily returns to his breeding grounds, only to reappear the following winter. This raptor has associated that particular tree branch with easy food and distinguishes it from millions of virtually identical tree branches between Scandinavia and Flanders. Linking sensory stimuli with value is essential to survival, not only for this particular bird but for any organism, both invertebrates and vertebrates. This type of information needs to be encoded, stored, and quickly retrieved after the first stimulus–value associations have been made, and to remain available even after extensive periods of time in the absence of contingent stimulus–reward associations. While short-term memory allows recall of recently acquired information for up to a minute, its capacity is very limited. Long-term memory, on the other hand, has a much greater storage capacity, lasting up to a lifetime. The initial phase of stimulus–value associations depends on dopaminergic-dependent reward-prediction error signals broadcasted by the ventral midbrain, which can be either positive or negative. These signals are ideally suited to promote or suppress behavior associated with the reward (1, 2). The storage in short-term memory is thought to depend on the hippocampus and/or amygdala in the medial temporal lobe, in concert with several divisions of the frontal cortex (3). Over time, however, the memory becomes consolidated and is represented in a distributed cortical network independent of the hippocampus. Major, largely unanswered, questions relate to how and where item value is stored in long-term memory. In PNAS, Ghazizadeh et al. (4) address the “where” question by charting the brain network in nonhuman primates …

Researchers report trends in emissions of nitrogen oxides in the United States over the past decade. The results suggest challenges to meeting future air quality standards for ozone, according to the authors.